It's funny how things turn out sometimes. I had what I thought would be a good, solid game concept and tried to implement it on a Sinclair ZX81 in 16K, and, well it was a little bit more dull than I imagined it, but then as I was using the z88dk again, there were some compromises that I had to make mostly due to speed. Too much on the screen would have made proceedings too slow and boring anyway, so I decided to limit the maximum number of ships in a convoy, bombs and bullets. I also made the screen smaller and set a limit to the distance that each bullet will travel on the X and Y plane. I'm sure any pure Z80 coders could make a better implementation.
It's available from the RWAP Software website, and the source code is on pastebin here. Any comments, questions or feedback is welcome.